The section of road is closed in both directions and there is queuing traffic between J44 A4230 (Llansamlet) and J43 A465 (Llandarcy).
Cameras show a large amount of trapped traffic at the scene, and traffic monitors say an incident is believed to have happened between a car and a motorbike.
There is also slow traffic on the A465 Eastbound from M4 J43 (Llandarcy) to A474 (Neath Interchange).
Congestion has now built further and delays of around 35 minutes are expected on Fabian Way.
Traffic monitors say traffic on Fabian Way into Swansea is also struggling as traffic looks to divert.
According to traffic monitors, a large amount of traffic is trapped at the scene at the moment.
If you’re travelling in the area, a diversion is now in place via the A4230 through Skewen and the A465 past Neath.
Speaking about the incident, South Wales Police said: “We’re at the scene of a serious road traffic collision on the M4 near junction 43 (Llandarcy).
“The road is closed in both directions. We’re asking people to avoid the area if possible. Thank you for your patience.”
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